In recent years, the Internet has been widely spread, and a user can access various information on websites operated in various locations in the world and can thereby acquire the information. Accordingly, apparatuses capable of performing broadband access such as ADSL (Asymmetric Digital Subscriber Line) and FTTH (Fiber To The Home) have also begun to be rapidly spread.
IEEE Std 802.3ah (registered trademark)—2004 (Non-Patent Literature 1) discloses one system of a passive optical network (PON) which is medium-sharing communication where a plurality of optical network units (ONUs) perform data transmission with an optical line terminal (OLT) by sharing an optical communication line. Namely, there are defined EPON (Ethernet (registered trademark) PON) where all information including user information passing through a PON and control information for managing and operating the PON is communicated in Ethernet (registered trademark) frame format; and an EPON access control protocol (MPCP (Multi-Point Control Protocol)) and an OAM (Operations Administration and Maintenance) protocol. By exchanging MPCP frames between the optical line terminal and an optical network unit, subscription, unsubscription, multiple upstream access control, and the like, of the optical network unit are performed. In addition, Non-Patent Literature 1 describes a method of registering a new optical network unit, REPORT indicating a bandwidth allocation request, and GATE indicating a transmission instruction, which use MPCP messages.
Note that in 10G-EPON, too, i.e., EPON with a communication speed equivalent to 10 gigabits/second, which is standardized in IEEE802.3av (registered trademark)—2009 as the next generation technology of GE-PON (Giga Bit Ethernet (registered trademark) Passive Optical Network) which is EPON that achieves a communication speed of 1 gigabit/second, an access control protocol is premised on MPCP.
Meanwhile, to implement long-distance optical transmission in a PON system, etc., for example, an avalanche photodiode (APD) with high light receiving sensitivity is used as a light receiving element. The APD is a light receiving element having current amplifying action. In order for the APD to accurately maintain a carrier multiplication factor, i.e., multiplication factor, having a strong temperature dependence, a reverse bias voltage applied to the APD needs to be adjusted according to the temperature of the APD.
For a technique for adjusting the reverse bias voltage applied to the APD, for example, Japanese Unexamined Patent Publication No. 2009-105489 (Patent Literature 1) discloses a configuration as shown below. Specifically, a transmission/reception integral type optical transceiver that transmits and receives optical signals includes an optical transmission sub-assembly including a laser diode; an optical reception sub-assembly including an avalanche photodiode; a first control circuit that has a temperature sensor mounted thereon and that drives the laser diode based on an output signal from the temperature sensor; and a second control circuit that controls a bias voltage of the avalanche photodiode. The second control circuit controls the bias voltage based on a temperature correction value which is obtained by adding a predetermined offset value to an output from the temperature sensor.